Vapor phase deposition of organic films

ABSTRACT

Methods and apparatus for vapor deposition of an organic film are configured to vaporize an organic reactant at a first temperature, transport the vapor to a reaction chamber housing a substrate, and maintain the substrate at a lower temperature than the vaporization temperature. Alternating contact of the substrate with the organic reactant and a second reactant in a sequential deposition sequence can result in bottom-up filling of voids and trenches with organic film in a manner otherwise difficult to achieve.

REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.17/820,180, filed Aug. 16, 2022, which is a continuation of U.S.application Ser. No. 16/877,129, filed May 18, 2020, now U.S. Pat. No.11,446,699, which is a continuation of U.S. application Ser. No.14/879,962, filed Oct. 9, 2015, now U.S. Pat. No. 10,695,794, thedisclosures of each of which are incorporated herein by reference intheir entireties and for all purposes.

BACKGROUND Field

The present invention relates to forming organic thin films by vapordeposition.

Description of the Related Art

Organic thin films have valuable optical, thermal, electrical andmechanical properties and are widely used in the electronics, medicalengineering, defense, pharmaceutical, and micro- and nanotechnologyindustries. Polymers in the microelectronics and photonics industriesinclude, among other examples, photon- or electron-curable/degradablepolymers for lithographic patterning; and polyimides for packaging,interlayer dielectrics and flexible circuit boards. Norrman et al.,Annu. Rep. Prog. Chem., Sect. C, 2005, 101, 174-201.

Polyimide films in particular are valuable for their thermal stabilityand resistance to mechanical stress and chemicals. Polyimide thin filmscan be used as a starting point in semiconductor applications foramorphous carbon films or layers, which are needed for future V-NANDstructures. Polyimide films can be used, for example, as antireflectionlayers to improve pattern definition and reduce misalignment inlithography steps, as layers in multiple patterning (e.g., SDDP, SDQP),as insulating materials for interlayer dielectric materials, or as thegate dielectric in all-organic thin film transistors.

Polymer thin films have traditionally been fabricated throughspin-coating techniques. The spin-coating method forms highly functionalpolymer films by coating a rotating disc with a liquid material andsintering the liquid. However, tailoring of spin-applied films islimited for several reasons. For instance, formation of uniform thinfilms on a substrate is difficult to control, in part because of theviscosity of the starting liquid, and it can be difficult to fill thegaps of very small features (e.g., trenches or gaps between metal lines)without void generation after curing. Also, spin-coating over hightopography relative to the desired thickness of the layer can result indiscontinuous and non-conformal deposition. As semiconductor chip sizescontinue to shrink, thinner and higher-strength films with more tunablemorphology are required.

Recently, vapor phase deposition processes such as chemical vapordeposition (CVD), vapor deposition polymerization (VDP), molecular layerdeposition (MLD), and sequential deposition processes such as atomiclayer deposition (ALD) and cyclical CVD have been applied to theformation of polymer thin films. In CVD, a film is deposited whenreactants react on a substrate surface. Gases of one or more reactantsare delivered to one or more substrates in a reaction chamber. Inthermal CVD, reactant gases react with one another on a hot substrate toform thin films, with the growth rate influenced by the temperature andthe amount of reactant supplied. In plasma enhanced CVD, one or morereactants can be activated in a remote plasma generator or in situ. InALD, a film is built up through self-saturating surface reactionsperformed in cycles. Vapor phase reactants are supplied, alternatinglyand repeatedly, to the substrate or wafer to form a thin film ofmaterial on the wafer. In a typical process, one reactant adsorbs in aself-limiting process on the wafer. A different, subsequently pulsedreactant reacts with the adsorbed species of the first reactant to formno more than a single molecular layer of the desired material. Thickerfilms are produced through repeated growth cycles until the targetthickness is achieved. Plasma enhanced variants of ALD, and hybridALD/CVD processes (e.g., with some overlaps of the reactants permitted)are also known.

SUMMARY OF THE INVENTION

In one aspect, a method is provided for depositing an organic film byvapor deposition. The method comprises vaporizing a first organicreactant in a vaporizer at a temperature A to form a first reactantvapor. A substrate in a reaction space is exposed to the first reactantvapor at a temperature B, which is lower than the temperature A at whichthe first organic reactant was vaporized. An organic film is depositedon the substrate.

In some embodiments, the organic film comprises a polymer. In someembodiments the polymer is a polyimide. In some embodiments, the organicfilm comprises polyamic acid. In some embodiments, the polyamic acid isfurther converted to polyimide. In some embodiments, the first organicreactant is a solid at room temperature and atmospheric pressure. Insome embodiments, the first organic reactant is a dianhydride, and moreparticularly, in some embodiments, PMDA.

The ratio of temperature A to temperature B in Kelvin is greater than 1.In some embodiments, the ratio of temperature A to temperature B inKelvin can be less than 1.8, between about 1 and 1.25, between about1.01 and 1.10, and/or between any of the other foregoing values.

In some embodiments, the temperature A can be greater than 120° C., lessthan 200° C., between about 120° C. and 250° C., between about 140° C.and 190° C., and/or between any of the other foregoing values.

In some embodiments, the temperature B is between about 5° C. and about50° C. lower than the temperature A, between about 10° C. and about 30°C. lower than the temperature A, and/or between any of the otherforegoing values lower than the temperature A.

In some embodiments, the temperature B can be greater than 20° C., lessthan 250° C., between about 20° C. and 250° C., between about 100° C.and 200° C., between about of 120° C. to 180° C., and/or between any ofthe other foregoing values.

In some embodiments, the method further includes removing excess of thefirst reactant vapor from contact with the substrate. The substrate isthen exposed to a second reactant, such that the first reactant vaporand the second reactant vapor do not substantially mix, and excess ofthe second reactant is removed from contact with the substrate. In someembodiments, the steps of exposing the substrate to the first reactantvapor and exposing the substrate to the second reactant are repeated ina plurality of cycles, such that the first reactant vapor and the secondreactant vapor do not substantially mix. In some embodiments, the secondreactant is a diamine, and more particularly, in some embodiments,1,6-diaminohexane (DAH). In some embodiments, each of removing theexcess of the first reactant vapor and removing the excess of the secondreactant vapor occurs over a time period greater than 1 second, lessthan 10 seconds, between about 1 second and about 10 seconds, and/orbetween any of the other foregoing values.

In some embodiments, when the first reactant vapor is exposed to thesubstrate, it is transported from the vaporizer to the reaction spacethrough a gas line. In some embodiments, the gas line is at atemperature C, which is higher than the temperature A at which the firstorganic reactant was vaporized.

In some embodiments, the substrate comprises a non-planar topography,and the deposited organic film comprises forming a first thickness on alower feature of the substrate, and depositing a second thickness on anupper field region of the substrate, where the first thickness isgreater than the second thickness.

In another aspect, a method is provided for controlling planarity of adeposited organic film. The method comprises vaporizing a first organicreactant in a vaporizer at a temperature A to form a first reactantvapor; exposing a substrate in a reaction space to the first reactantvapor at a temperatures B, which is lower than the temperature A; andremoving excess of the first reactant vapor from contact with thesubstrate over a period of time, where decreasing the period of timeincreases the planarity of the deposited organic film. In someembodiments the deposited organic film has thickness non-uniformity(1sigma) of below about 20%, below about 10%, below about 5%, belowabout 2%, below about 1% and below about 0.5%. In some embodiments thesubstrate is a semiconductor wafer, such as 200 mm or 300 silicon mmwafer, or a glass substrate.

In some embodiments, the method further comprises exposing the substrateto a second reactant such that the first reactant vapor and the secondreactant do not substantially mix; removing excess of the secondreactant from contact with the substrate; and repeating exposure of thesubstrate to the first reactant vapor and exposure of the substrate tothe second reactant in a plurality of cycles, such that the firstreactant vapor and the second reactant do not substantially mix.

In another aspect, an apparatus for organic film deposition comprises avessel configured for vaporizing a first organic reactant to form afirst reactant vapor, a reaction space configured to accommodate asubstrate and in selective fluid communication with the vessel; and acontrol system. In a preferred embodiment, the control system isconfigured to maintain the reactant in the vessel at or above atemperature A, maintain the substrate at a temperature B that is lowerthan the temperature A, transport the first reactant vapor from thevessel to the substrate, and deposit an organic film on the substrate.

In some embodiments, the apparatus is configured to deposit a polymer.In some embodiments, the polymer comprises a polyimide. In someembodiments, the apparatus is configured to deposit polyamic acid. Insome embodiments, the polyamic acid can be converted to polyimide.

In some embodiments, the apparatus further comprises a gas line fluidlyconnecting the vessel to the reaction space, wherein the control systemis further configured to maintain the gas line at a temperature C thatis higher than the temperature A.

In some embodiments, the control system is further configured totransport a second reactant vapor to the substrate alternately with thefirst reactant vapor in a sequential deposition process.

In some embodiments, the apparatus further comprises an outlet line andan inert gas source connected to the reaction space, and the controlsystem is further configured to remove excess reactant vapors andbyproduct between supply of the first reactant vapor and the secondreactant vapor.

In another aspect, a method for reducing the aspect ratio ofthree-dimensional structures on a substrate is provided. The methodincludes vaporizing a first reactant to form a first reactant vapor. Thesubstrate is exposed in a reaction space to the first reactant vapor,the substrate that includes a topography with a three-dimensionalstructure. An organic film is deposited over the substratepreferentially over lower features of the topography compared to higherfeatures of the topography such that the organic film reduces an aspectratio of the three-dimensional structure on the substrate as itdeposits. Depositing includes exposing the substrate to the firstreactant vapor.

In another aspect, a method is provided for forming an organic film. Themethod includes vaporizing a first reactant in a vaporizer to form afirst reactant vapor. A substrate in a reaction space is exposed to thefirst reactant vapor and a second reactant vapor. A polyamic acid filmfrom the first reactant vapor and the second reactant vapor on thesubstrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B are flow diagrams illustrating methods for vapor depositionof an organic film.

FIGS. 2A-2D are schematic representations of examples of vapordeposition apparatuses that can be employed for the deposition processesdescribed herein.

FIGS. 3A-3B are graphs illustrating temperature at different stages ofmethods for vapor depositing an organic film.

FIGS. 4A-4E are representations of bottom-up filling of trenches inaccordance with a method for vapor depositing an organic film.

FIGS. 5A-5D are thickness maps of films deposited by methods in whichthe deposition temperature is higher than the vaporization vessel and bya deposition process employing higher vaporization temperatures thandeposition temperatures, respectively.

FIGS. 6A-6B are representations of bottom-up filling of trenches inaccordance with a method for vapor depositing an organic film.

DETAILED DESCRIPTION OF EMBODIMENTS

Vapor phase deposition techniques can be applied to organic films andpolymers such as polyimide films, polyamide films, polyurea films,polyurethane films, polythiophene films, and more. CVD of polymer filmscan produce greater thickness control, mechanical flexibility, conformalcoverage, and biocompatibility as compared to the application of liquidprecursor. Sequential deposition processing of polymers can produce highgrowth rates in small research scale reactors. Similar to CVD,sequential deposition processes can produce greater thickness control,mechanical flexibility, and conformality. The terms “sequentialdeposition” and “cyclical deposition” are employed herein to apply toprocesses in which the substrate is alternately or sequentially exposedto different precursors, regardless of whether the reaction mechanismsresemble ALD, CVD, MLD or hybrids thereof.

However, vapor phase deposition of organic thin films can be challengingfor a variety of reasons. For example, reactants for fabricating organicfilms tend to have low vapor pressure and volatility, and thus require ahigh source temperature to vaporize. It can be difficult to ensuresufficient vapor pressure is developed to allow for the vapor depositionto properly proceed, while at the same time avoiding thermaldecomposition. Furthermore, the substrate temperature is typicallyhigher than the vaporizer to drive the deposition reactions, but highvaporization temperatures to increase the vapor pressure of theprecursor not only risks premature thermal decomposition, but also canlead to excessively high deposition rates and consequent non-conformaldeposition.

For example, polyimide film can be deposited by reacting a dianhydrideand a diamine, and the dianhydride typically used for this process ispyromellitic dianhydride (PMDA). At room temperature and atmosphericpressure, PMDA is a solid with quite low vapor pressure, andconsequently, it requires heating to vaporize. Failure to controlevaporation temperatures in CVD/VDP of polyimide films can lead to crackformation, and, despite potential on the small research scale,production-scale sequential deposition of polyimide faces numerousdifficulties for manufacturability (e.g., particles, poor repeatability,clogging of gas lines, poor uniformity, low growth rate).

Due to strict requirements of reactant volatility and growthtemperature, obtaining high quality organic films using conventionalvapor phase deposition techniques is challenging. Accordingly, a needexists for an improved approach for vapor deposition of organic thinfilms.

In embodiments described herein, the growth temperature at the substratecan be lower than the reactant source temperature. This temperatureprofile allows high enough vapor pressure for the reactant (e.g.,precursors for organic film deposition, such as PMDA) to vaporize, lowenough growth temperature to avoid the problems of overheating, andenables a high growth rate process. Deposition processes taught hereincan achieve high growth rate and throughput, and produces high qualityorganic thin films.

FIG. 1A is a simplified flow diagram of a method for vapor deposition ofan organic film. In the first illustrated block 10, a first organicreactant is vaporized at a temperature A to form a first reactant vapor.The reactant being vaporized may be liquid or solid under standardtemperature and pressure conditions (room temperature and atmosphericpressure). In some embodiments, the reactant being vaporized comprisesan organic precursor, such as a dianhydride, for example pyromelliticdianhydride (PMDA). In block 20, the substrate is exposed to the firstreactant vapor at a temperature B that is lower than the temperature A,and in block 30, an organic film deposited. The method can includeadditional steps, and may be repeated, but need not be performed in theillustrated sequence nor the same sequence in each repetition ifrepeated, and can be readily extended to more complex vapor depositiontechniques.

In some embodiments, the organic film comprises a polymer. In someembodiments, the polymer deposited is a polyimide. In some embodiments,the polymer deposited is a polyamide. In some embodiments, the polymerdeposited is a polyurea. Other examples of deposited polymers includedimers, trimers, polyurethanes, polythioureas, polyesters, polyimines,other polymeric forms or mixtures of the above materials.

In some embodiments, the organic film comprises a precursor material toa polymer film that can be converted or polymerized by a treatmentprocess. For example, the as-deposited organic film comprise a polyamicacid. In some embodiments, the polyamic acid is converted to apolyimide. In particular, polyamic acid is a common polyimide precursorthat can be cyclized, or imidized, to form polyimide. It has been foundin experiments that, for vapor deposition using a dianhydride anddiamine, the composition of the as-deposited film depends upon thesubstrate temperature. For example, in experiments, below about 130° C.the as-deposited film was found to be mostly polyamic acid. Betweenabout 130° C. and 160° C., the film was a mixture of polyamic acid andpolyimide. Above about 160° C. the film was mostly polyimide (polymer).Polyamic acid can be converted to polyimide in a variety of techniques,including annealing, plasma (e.g., using an inert or rare gas), chemicaltreatment (e.g., using an anhydride), UV treatment, and otherpost-deposition treatments.

The term “about” is employed herein to mean within standard measurementaccuracy.

The techniques taught herein can be applied to vapor depositiontechniques, including CVD, VPD, ALD, and MLD in a wide variety ofreactor configurations. FIG. 1B is a simplified flow diagram of asequential deposition process, and FIGS. 2A-2D illustrate schematicrepresentations of exemplary reactor configurations.

The flow chart of FIG. 1B illustrates a sequential deposition method forvapor deposition of an organic film. In block 10, a first organicreactant is vaporized at temperature A to form a first reactant vapor.In block 40, the first reactant vapor is transported to the substratethrough a gas line at temperature C, which is higher than temperature A.In an embodiment, the first reactant, or species thereof, chemicallyadsorbs on the substrate in a self-saturating or self-limiting fashion.The gas line can be any conduit that transports the first reactant vaporfrom the source to the substrate. In block 20, the substrate is exposedto the first reactant vapor at a temperature B that is lower than thetemperature A. In block 45, excess of the first reactant vapor (and anyvolatile reaction by-product) is removed from contact with thesubstrate. Such removal can be accomplished by, e.g., purging, pumpdown, moving the substrate away from a chamber or zone in which it isexposed to the first reactant, or combinations thereof. In block 50, thesubstrate is exposed to a second reactant vapor. In an embodiment, thesecond reactant may react with the adsorbed species of the firstreactant on the substrate. In block 60, excess of the second reactantvapor (and any volatile reaction by-product) is removed from contactwith the substrate, such that the first reactant vapor and the secondreactant vapor do not mix. In some embodiments the vapor depositionprocess of the organic film does not employ plasma and/or radicals, andcan be considered a thermal vapor deposition process.

Various reactants can be used for these processes. For example, in someembodiments, the first reactant is an organic reactant such as ananhydride, for example a dianhydride, e.g., pyromellitic dianhydride(PMDA), or any other monomer with two reactive groups. In someembodiments, the first reactant can be an anhydride, such asfuran-2,5-dione (maleic acid anhydride). In some embodiments, the secondreactant is also an organic reactant capable of reacting with adsorbedspecies of the first reactant under the deposition conditions. Forexample, the second reactant can be a diamine, e.g., 1,6-diamnohexane(DAH), or any other monomer with two reactive groups which will reactwith the first reactant. In some embodiments, different reactants can beused to tune the film properties. For example, a polyimide film and/orpolyimide precursor material (e.g., polyamic acic) film could bedeposited using 4,4′-oxydianiline or 1,4-diaminobenzene instead of1,6-diaminohexane to get a more rigid structure with more aromaticityand increased dry etch resistance. In some embodiments the reactants donot contain metal atoms. In some embodiments the reactants do notcontain semimetal atoms. In some embodiments one of the reactantscomprises metal or semimetal atoms. In some embodiments the reactantscontain carbon and hydrogen and at least one or more of the followingelements: N, O, S, P or a halide, such as Cl or F. Deposition conditionscan differ depending upon the selected reactants and can be optimizedupon selection. For sequential deposition of polyimide using the PMDAand DAH in a single wafer deposition tool, substrate temperatures can beselected from the range of about 100° C. to about 250° C., and pressurescan be selected from the range of about 1 mTorr to about 760 Torr, moreparticularly between about 100 mTorr to about 100 Torr. In someembodiments, the reactant being vaporized comprises an organic precursorselected from the group of 1,4-diisocyanatobutane or1,4-diisocyanatobenzene. In some embodiments the reactant beingvaporized comprises an organic precursor selected from the group ofterephthaloyl dichloride, alkyldioyl dichlorides, such as hexanedioyldichloride, octanedioyl dichloride, nonanedioyl dichloride, decanedioyldichloride, or terephthaloyl dichloride. In some embodiments, thereactant being vaporized comprises an organic precursor selected fromthe group of 1,4-diisothiocyanatobenzene or terephthalaldehyde. In someembodiments, the reactant being vaporized can be also diamine, such as1,4-diaminobenzene, decane-1,10-diamine, 4-nitrobenzene-1,3-diamine or4,4′-oxydianiline. In some embodiments, the reactant being vaporized canbe terephthalic acid bis(2-hydroxyethyl) ester. In some embodiments thereactant being vaporized can be carboxylic acid, for example alkyl-,alkenyl-, alkadienyl-dicarboxylic or tricarboxylic acids, such asethanedioic acid, propanedioic acid, butanedioic acid, pentanedioic acidor propane-1,2,3-tricarboxylic acid. In some embodiments, the reactantbeing vaporized can be aromatic carboxylic or dicarboxylic acid, such asbenzoic acid, benzene-1,2-dicarboxylic acid, benzene-1,4-dicarboxylicacid or benzene-1,3-dicarboxylic acid. In some embodiments, the reactantbeing vaporized can be selected from the group of diols, triols,aminophenols such as 4-aminophenol, benzene-1,4-diol orbenzene-1,3,5-triol. In some embodiments, the reactant being vaporizedcan be 8-quinolinol. In some embodiments, the reactant being vaporizedcan comprise alkenylchlorosilanes, like alkenyltrichlorosilanes, such as7-octenyltrichlorosilane

In block 30, an organic film is deposited. The skilled artisan willappreciate that block 30 may represent the result of blocks 10, 40, 20,45, 50 and 60, rather than a separate action. The blocks 10-60 togetherdefine a cycle 70, which can be repeated until a film of sufficientthickness is left on the substrate (block 80) and the deposition isended (block 90). The cycle 70 can include additional steps, need not bein the same sequence nor identically performed in each repetition, andcan be readily extended to more complex vapor deposition techniques. Forexample, cycle 70 can include additional reactant supply blocks, such asthe supply and removal of additional reactants in each cycle or inselected cycles. Though not shown, the process may additionally comprisetreating the deposited film to form a polymer (e.g., UV treatment,annealing, etc.).

In some embodiments the organic film does not contain metal atoms. Insome embodiments the organic film does not contain semimetal atoms. Insome embodiments the organic film contains metal or semimetal atoms. Insome embodiments the organic film contains carbon and hydrogen and atleast one or more of the following elements: N, O, S, or P.

FIG. 2A is a simplified schematic representation of apparatus 100 forvapor deposition of an organic film. The apparatus includes a firstreactant vessel 105 configured for vaporizing a first organic reactant110 to a first reactant vapor. A reaction chamber defines a reactionspace 115 configured to accommodate at least one substrate 120. Acontrol system 125 is configured to maintain the first reactant 110 inthe first reactant vessel 105 at a temperature A, and is configured tomaintain the substrate 120 in the reaction space 115 at a temperature B,where the temperature B is lower than the temperature A.

A gas line 130 fluidly connects the first reactant vessel 105 to thereaction space 115, and is configured to selectively transport the firstreactant vapor from the first reactant vessel 105 to an inlet manifold135 to the reaction space 115. In an embodiment, the control system 125or a separate temperature control is configured to maintain the gas line130 at a temperature C, where the temperature C is higher than thetemperature A.

The apparatus 100 includes a second reactant vessel 140 holding a secondreactant 145. In some embodiments, the second reactant 145 is naturallyin a gaseous state; in other embodiments, the second reactant vessel 140is also configured to vaporize the second reactant 145 from a naturalliquid or solid state. The second reactant vessel is in selective fluidcommunication with the inlet manifold 135. The inlet manifold caninclude a shared distribution plenum across the chamber width, or canmaintain separate paths to the reaction space 120 for separatereactants. For sequential deposition embodiments, it can be desirable tokeep the reactant inlet path separate until introduction to the reactionspace 115 in order to avoid reactions and along the surface of commonflow paths for multiple reactants, which can lead to particlegeneration. The apparatus can in some embodiments include additionalvessels for supply of additional reactants.

One or more inert gas source(s) 150 is (are) in selective fluidcommunication with the first reactant vessel 105 and with the reactionspace 115. The inert gas source 150 can also be in selective fluidcommunication with the second reactant vessel 140, as shown, and anyother desired reactant vessels to serve as a carrier gas. The controlsystem 125 communicates with valves of the gas distribution system inaccordance with deposition methods described herein. For sequentialdeposition processing, the valves are operated in a manner thatalternately and repeatedly exposes the substrate to the reactants,whereas for simultaneous supply of the reactants in a conventional CVDprocess, the valves can be operated to simultaneously expose thesubstrate to mutually reactive reactants.

An exhaust outlet 155 from the reaction space 115 communicates throughan exhaust line 160 with a vacuum pump 165. The control system 125 isconfigured to operate the vacuum pump 165 to maintain a desiredoperational pressure and exhaust excess reactant vapor and byproductthrough the exhaust outlet 155.

FIG. 2B schematically illustrates an example of a showerhead reactionchamber 200 that can be employed for vapor deposition of an organic filmas described herein. The reactor includes a showerhead 204 configured byreceive and distribute reactant vapors across a substrate 206 on asubstrate support 208. While illustrated as a single substrate chamber,the skilled artisan will appreciate that shower reactors can alsoaccommodate multiple substrates. A reaction space 209 is defined betweenthe showerhead 204 and the substrate 206. A first inlet 210 communicateswith a source of a first reactant, and a second inlet 212 communicateswith a source of a second reactant. Additional inlets (not shown) can beprovided for separate sources of inert gases and/or additionalreactants, and the showerhead 204 can also be provided with a separateexhaust (not shown) to speed removal of reactants between phases forsequential deposition (e.g., ALD) processes. While the first inlet 210and the second inlet 212 are both shown communicating with a singleplenum of the showerhead 204, it will be understood that in otherarrangements the inlets can independently feed reactants to the reactionspace and need not share a showerhead plenum. An exhaust outlet 214,shown in the form of an exhaust ring surrounding the base of thesubstrate support 208, communicates with a vacuum pump 216.

FIG. 2C illustrates a different configuration of a reaction chamber 230that can be employed for vapor deposition of an organic film asdescribed herein, where features similar in function to those of FIG. 2Bare referenced by like reference numbers. Typically known as ahorizontal flow reactor, the reaction chamber 230 is configured with afirst reactant inlet 210 and a second reactant inlet 212, and an exhaustoutlet 216. While illustrated as a single substrate chamber, the skilledartisan will appreciate that horizontal flow reactors can alsoaccommodate multiple substrates. Additional inlets (not shown) can beprovided for separate sources of inert gases and/or additionalreactants. Separate inlets 210, 212 are shown to minimize depositionreactions upstream of the reaction space 209, as is generally preferredfor sequential deposition reactors, but it will be understood that inother arrangements the different reactants can be provided through acommon inlet manifold, particularly for CVD processing. While the secondinlet 212 is illustrated as feeding from a remote plasma unit 202, theskilled artisan will appreciate that the RPU can be omitted or leftunpowered for thermal deposition processes. The skilled artisan willappreciate that in other types of horizontal flow reactors, thedifferent reactants can also be provided from different sides of thechamber, with separate exhausts operated alternately on the differentsides, such that a first reactant can flow in one direction and a secondreactant can flow in another direction in separate pulses.

FIG. 2D illustrates another example of a reaction chamber 240 that canbe employed for vapor deposition of an organic film. The illustratedchamber is configured for space-divided sequential deposition reactions,rather than time-divided reactions. The space-divided reactions employdifferent zones, here zones A, B, C and D, through which substratesmove. Alternatively, the gas injection system can move in relation tothe substrates and substrates might be stationary or rotating. The zonesare separated by barriers 242, which may be physical walls, inert gascurtains, exhausts, or combinations thereof that minimize vaporinteractions among the zones A-D. The substrate support(s) 208 can takethe form of a rotating platform, as shown, or a conveyor belt (notshown) for linearly arrayed zones. In one example, zone A could beplumbed and operated to be supplied consistently with a first reactant,such as a precursor that adsorbs on the substrate, zones B and D couldbe plumbed and operated to be supplied with inert or purge gas, and zoneC could be plumbed and operated supplied with a second reactant thatreacts with the adsorbed species of the first reactant. Substrates 206(four shown) move through the zones to sequentially be exposed to thefirst reactant (zone A), inert gas (zone B), second reactant (zone C),and inert gas (zone D) before the cycle is repeated. In the case ofspace-divided plasma sequential deposition, the residence time of thereactants can depend on both the speed of the reactants through the zoneas well as the rate of movement of the substrate support 208. In somecases the substrate is stationary or rotating and the gas supply system,such as gas injector(s), is rotated over the substrates. Rotation speedof the injector(s) or substrates can also affect the gas residence time.In variations on space-divided sequential deposition, a combination ofspace-divided and time-divided sequential deposition could supplydifferent reactants at different times to the same zone, whilesubstrates move through the zones. Each zone may supply separatereactants, and additional zones may be added by providing largerplatforms divided by greater numbers of zones, or by providing longerconveyors through greater numbers of zones.

While not shown, the skilled artisan will readily appreciate that theprinciples and advantages taught herein are applicable to other types ofvapor deposition reactors, including batch reactors, such as verticalfurnaces, which are known in the art for CVD and sequential deposition(e.g., ALD, cyclical CVD and hybrids) processing.

The graphs of FIGS. 3A-3B illustrate the temperature at different stagesof methods for vapor depositing an organic film. FIG. 3A illustrates atemperature profile along the reactant path in accordance withembodiments. The source of the reactant is vaporized at a temperature A.The reaction chamber, or at least the substrate, is kept at atemperature B, which is lower than the temperature A. FIG. 3Billustrates the temperature profile of some embodiments where thereactant vapor is transported from the vaporization vessel to thereaction chamber in a gas line at a temperature C that is higher thanthe temperature A. The higher temperature gas line reduces the risk ofcondensation and consequent contamination and/or gas line clogging.

The illustrated temperature profile can be applied to a wide variety ofvapor deposition processes that involve low vapor pressure reactantsand/or growth temperature restrictions. The particular temperatures ineach reaction will depend on multiple factors, including the reactants,desired film properties, deposition mechanism and reactor configuration.The embodiments are particularly useful for vaporizing organicprecursors for vapor phase organic film deposition.

Precursor condensation or multilayer adsorption can cause problems inrepeatability and process stability. Condensation or multilayeradsorption can occur when the source temperature is higher than thedeposition temperature. In some embodiments, the pressure in the sourcevessel and source lines is higher than the pressure in the reactionchamber or zone where deposition takes place. This negative pressuredifference can decrease the probability of precursor condensation andmultilayer adsorption. This negative pressure difference can be appliedto one or more of the reactants to a vapor deposition process, includingboth reactants subject to the temperature profile illustrated in FIG. 3Aand reactants not subject to the temperature profile illustrated in FIG.3A. In experiments, the PMDA source line was at 45-50 Torr while thereaction chamber was at about 2-10 Torr. In some embodiments, thepressure difference between the source line and the reaction chamber orzone where deposition takes place can be greater than 1 mTorr, less than760 Torr, between about 1 mTorr and 760 Torr, between about 5 mTorr and300 Torr, between about 10 Torr and 200 Torr, and/or between any of theother foregoing values. In some embodiments the ratio of the pressure ofthe source line to the pressure of the reaction chamber or zone wheredeposition takes place, in Torr, can be greater than 1.01, less than1000, between about 2 and 100, between about 3 and 50, between about 5and 25, and or between any of the other foregoing values.

In some embodiments of the invention, the temperature A can be greaterthan 120° C., less than 250° C., between about 120° C. and 200° C.,between about 140° C. and 190° C., and/or between any of the otherforegoing values. In some embodiments, the temperature B is betweenabout 5° C. and about 50° C. lower than the temperature A, between about10° C. and about 30° C. lower than the temperature A, and/or between anyof the other foregoing values lower than the temperature A. In someembodiments, the temperature C is between about 0.1° C. and about 300°C. higher than the temperature A, between about 1° C. and about 100° C.higher than the temperature A, between about 2° C. and about 75° C.higher than the temperature A, between about 2° C. and about 50° C.higher than the temperature A, and/or between any of the other foregoingvalue higher than the temperature A. In some embodiments, the ratio oftemperature C to temperature A in Kelvin is between about 1.001 andabout 2.0, between about 1.001 and about 1.5, between about 1.001 andabout 1.25 and/or between about 1.001 to about 1.10. In some embodimentsthe temperature C can be lower than temperature A, but higher thantemperature B. In some embodiments the temperature C can be betweenabout 0.1° C. to about 200° C., between about 0.1° C. to about 50° C.,between about 0.1° C. to about 30° C. lower than temperature A, buthigher than temperature B. However in some embodiments the temperature Ccan be about the same as temperature A, but higher than temperature B.In some embodiments the temperatures A, B and C can be about equal

In addition to the low vapor pressure of reactants, the fine particulateform of solid reactants can pose problems during vapor deposition. Theparticles can be easily blown or carried to the substrate, for example,if the pressure differences during pulsing for deposition are too great.While filters can be used to reduce the particulates blown or carried tothe substrate, filters can become clogged, and can decrease the gas lineconductance so much that the dose becomes too low. Accordingly it ispreferable to limit the pressure differences during deposition to lessthan about 80 Torr, and more particularly to less than about 50 Torr,and do without filters.

It has been found that depositing organic film using the embodimentsdescribed herein facilitates tailoring film morphology. In someembodiments, employing alternate pulsing to reactants and equipment andlower deposition temperature compared to the precursor source vessel, orvaporizer, a desirably non-conformal film that reduces the aspect ratioof three-dimensional structures can be deposited on a non-planarsubstrate. In some embodiments, the non-planar substrate comprisestrenches or vias or other three-dimensional structures. The film can bedeposited in a manner that achieves thicker film on a lower feature ofthe substrate than on an upper field region of the substrate. Suchbottom-up deposition is surprising given that conventional vapordeposition typically either grows faster on upper field areas (such asconventional CVD), leading to pinching at the top of trenches and“keyhole” formation, or is conformal (such as conventional sequentialdeposition processes).

FIGS. 4A-4C are schematic representations of a vapor deposition processthat reduces the aspect ratio of three-dimensional structures of asubstrate in accordance with some embodiments. FIG. 4A illustrates aschematic representation of a cross section of a substrate 400 with apattern of three dimensional (3D) features in the form of trenches 410.In other embodiments, the substrate can have different surfacetopography. The 3D features can be quite small with high aspect ratios,which ordinarily makes it difficult to reach the bottom with depositionand fill gaps in the features, or trenches, without forming voids. Inthe illustrated embodiment, the 3D features can have lateral dimensionsfrom 5 nm to 10 μm, more particularly about 5 nm to about 500 nm, orabout 10 nm to about 200 nm. At the same time, the ratio of height towidth, or aspect ratio, of the 3D features, or trenches 410 for theillustrated embodiment, can range between about 0.25 to 1000, about 0.5to about 100, more particularly about 1.0 to 75, and even moreparticularly from about 2.0 to about 50. FIG. 4B illustrates a crosssection of the substrate 400 where the polymer 420 being depositedexhibits reduction of the aspect ratio of the trenches 410 as thedeposition favors the bottom of the 3D features in a bottom-up fillingprocess, in contrast to most vapor deposition techniques. FIG. 4Cillustrates a cross section of the substrate 400 where the depositedorganic film 420 has filled the trenches 410 evenly without any seamsvisible in the micrograph and without voids. In some embodiments, thedeposited organic film decreases the aspect ratio in thethree-dimensional structures by a factor more than about 1.5, more thanabout 5, more than about and more than about 25 or in some embodimentsby a factor more than about 100. In some embodiments, the depositedorganic film decreases the aspect ratio of the substrate so that thereis no substantial aspect ratio left anymore after the deposition of theorganic film. In some embodiments, the deposited organic fills thethree-dimensional structures, such as vias or trenches, at least about50%, at least about 75%, at least about 90%, at least about 95% of thevolume of the three-dimensional structure without having any substantialseam or voids in the filled volume. In some embodiments the depositedorganic fills the three-dimensional structures, such as vias ortrenches, fully and/or there exists organic and substantially planarfilm above top level of the three-dimensional structures in thesubstrate. The deposited organic film can comprise polyamic acid,polyimide, polyurea, polyurethane, polythophene, and mixtures thereof.

FIGS. 4D-4E are electron micrographs showing the results of a negativetemperature difference experiment, where PMDA and DAH were alternatelyand sequentially provided to the substrate in sequential depositionprocess to deposit a polyimide film. The first reactant PMDA wasvaporized at a temperature of 150° C., the PMDA gas line was maintainedat 155° C., and the substrate was maintained at 127° C. Line flows of450 sccm, pump line pressure of 2 torr, and source line pressure of40-100 torr were used. Pulse/purge lengths of 11/8.1 seconds and 4.7/9seconds were used for PMDA and DAH, respectively. FIG. 4D illustrates across section of a substrate 400 where a polymer 420 has been depositedwith bottom-up filling of the trenches 410 after 20 cycles. FIG. 4Eillustrates a cross section of a substrate 400 where a polymer 420 hasbeen deposited with bottom-up filling of the trenches 410 after 60cycles. The deposited film of FIG. 4E exhibits a relatively planarsurface compared to the topography of the initial trenches.

In some embodiments, planarity of the film can be tailored based on thelength of the time period over which excess of reactant vapor is removedfrom contact with the substrate. Decreasing the period of time overwhich excess reactant is removed increases the planarity of thedeposited organic film. In some embodiments, each of removing the excessof the first reactant vapor and removing the excess of the secondreactant vapor occurs over a time period greater than 1 second, lessthan 10 seconds, between about 1 second and about 10 seconds, and/orbetween any of the other foregoing values.

EXAMPLE 1

FIGS. 5A-5D show the results of experiments comparing similar sequentialdeposition processes using a negative temperature difference from thevaporizer to the substrate (FIGS. 5A & 5B) and using a positivetemperature difference from the vaporizer to the substrate (FIGS. 5C &5D). All experiments employed 300 mm wafers in a PULSAR 3000™ beta ALDtool supplied by ASM International, N.V. (Almere, The Netherlands). Thenegative temperature difference deposited a film at more than threetimes the growth rate, and produced a film with much higher thicknessuniformity, compared to a process with a positive difference.

For the negative temperature difference experiment, PMDA and DAH werealternately and sequentially provided to the substrate in a sequentialdeposition process to deposit a polyimide film. The first reactant PMDAwas vaporized at a temperature of 150° C., the PMDA gas line wasmaintained at 153° C., and the substrate was maintained at 127° C. Thesecond reactant DAH was kept at 45° C. Line flows of 450 sccm were used,and pulse/purge lengths of 11/8.066 seconds and 4.68/9 seconds were usedfor PMDA and DAH, respectively. The pulsing pressure difference was setto about 45 Torr for PMDA, and no line filters were used. 60 depositioncycles were applied, and the resulting film was analyzed byspectroscopic ellipsometry. FIGS. 5A & 5B show the thickness mapsobtained on a 200 mm wafer mapping size and a 300 mm wafer mapping size,respectively, in both cases employing 3 mm edge exclusions. The growthrate was 5.1 Å per cycle and 1σ thickness non-uniformities were 0.6% and1.4% using the 200 mm and 300 mm mapping sizes, respectively.

For the positive temperature difference experiment, the first reactantPMDA was vaporized at a temperature of 140° C., the PMDA gas line wasmaintained at 143° C., and the substrate was maintained at 150° C. Thesecond reactant DAH was kept at 45° C. Line flows of 450 sccm were used,and pulse/purge lengths of 5/5 seconds and 2/5 seconds were used forPMDA and DAH, respectively. The pulsing pressure difference was set toabout 45 Torr for PMDA, and no line filters were used. 165 depositioncycles were applied, and the resulting film was analyzed byspectroscopic ellipsometry. FIGS. 5C & 5D show the thickness mapsobtained using either 200 mm wafer mapping size and 300 mm wafer mappingsize, in both cases applying 3 mm edge exclusions. The growth rate was1.6 Å per cycle and 1σ thickness non-uniformities were 1.1% and 6.0%using the 200 mm and 300 mm mapping sizes, respectively.

EXAMPLE 2

In another negative temperature difference experiment conducted onwafers patterned with trenches, PMDA and DAH were reacted in asequential process to deposit a polyimide film on a substrate withtrench patterns. The trenches had variable pitches of 40 and 50 nm with25-35 nm openings. The first reactant PMDA was vaporized at atemperature of 150° C., the PMDA gas line was maintained at 153° C., andthe substrate was maintained at 127° C. The second reactant DAH was keptat 45° C. Line flows of 450 sccm were used, and pulse/purge lengths of11/8.066 seconds and 4.68/9 seconds were used for PMDA and DAH,respectively. The resulting film was analyzed by tunneling electronmicroscopy (TEM). After 20 cycles, the TEM image showed that the filmwas thicker on the trench bottom areas, and thinner on the side walls ofthe trenches. The film thickness on a planar wafer grown using the sameparameters was 7 nm, the film thickness on the bottom of some trencheswas about 11 nm, and the film thickness on the sides of some trencheswas about 4 nm. The growth was thus proceeding faster in the bottomareas of the trenches, indicating bottom-up filling. After 60 depositioncycles, the TEM analysis showed seamless, bottom-up gap filling of thetrenches with polyimide. The top surface was relatively smooth,exhibiting some self-planarizing behavior.

EXAMPLE 3

In another negative temperature difference experiment, PMDA and DAH werereacted in sequential deposition processes to deposit a polyimide filmson substrates with trench patterns. Different time purge lengths wereused. In one film, a purge length of 8.066 seconds was used for PMDA and9.0 seconds for DAH, in another film a purge length of 15 seconds wasused for each of PMDA and DAH, and in another film a purge length of 25seconds was used for each of PMDA and DAH. The resulting films wereanalyzed by TEM. Purge length did seem to affect gap fillingperformance. However, shorter purges resulted in more planar film on topof the structures. Purge length can thus be used as a factor to tailorthe final morphology of the film.

EXAMPLE 4

In another negative difference experiment, PMDA and DAH were reacted intwo separate alternative and sequential deposition processes atdifferent temperatures. In the first experiment, the PMDA was vaporizedat 150° C., and the substrate was maintained at 127° C. In the secondexperiment, the PMDA was vaporized at 180° C., and the substrate wasmaintained at 160° C. The film deposited in the first experiment waspredominantly polyamic acid, and the film deposited in the secondexperiment was predominantly polyimide. Deposition temperature appearsto affect the composition of the deposited film when the reactants arePMDA and DAH. A lower deposition temperature appears to lead to greaterproportion of polyamic acid, and a higher deposition temperature appearsto lead to greater proportion of polyimide.

EXAMPLE 5

In another negative temperature difference experiment, depositedpolyamic film was annealed to form polyimide. When reacting PMDA andDAH, polyamic acid is deposited in greater proportions at lowerdeposition temperatures. Conversion to polyimide was confirmed by FTIRspectroscopy. Data for the four polyamic films annealed at differenttemperature is as follows:

TABLE I Polyamic Film Deposited at 127° C. Annealed Film ThicknessThickness Ave. Non- Anneal Ave. Non- Thickness uniformity RefractiveTemp. Thickness uniformity Refractive Film (nm) (1σ) Index (° C.) (nm)(1σ) Index 1 32.898 1.44 1.578 200 22.707 1.99 1.6099 2 31.048 1.871.5719 250 20.438 2.89 1.6119 3 31.183 1.65 1.572 300 20.385 2.11 1.61494 30.665 1.81 1.5642 350 19.426 2.39 1.6056

EXAMPLE 6

In another negative temperature difference experiment, organic filmswere deposited at different temperatures. Thickness was analyzedthickness was measured with spectroscopic electrometry (SE) and X-rayreflectivity (XRR). Density and RMS-roughness were also measured. Datafor the four films is as follows:

TABLE II SE XRR Rough- Deposition Thickness Thickness Density ness FilmTemperature Anneal (nm) (nm) (g/cm³) (nm) 1 127° C. No 32.6 33.4 1.4190.338 2 127° C. 200° C. 24.6 24.6 1.434 0.449 3 150° C. No 25.2 25.91.472 0.377 4 160° C. No 38.2 39.4 1.401 0.400

EXAMPLE 7

In another negative temperature difference experiment, water was used toetch the deposited films to confirm conversion from polyamic acid to amore etch resistant polymer, such as polyimide. Polyamic acid is watersoluble and can be etched by water. Polyimide, by contrast, is not watersoluble and cannot be etched by water. The first film was deposited at127° C. and thus was predominantly polyamic acid. The second film wasdeposited at 160° C. and thus was predominantly polyimide. The thirdfilm was deposited at 127° C. and subsequently treated with argon plasmato convert the deposited polyamic acid to polyimide. Thickness of thefilms was measured before and after exposure to water and compared todetermine the extent of etching by the water. The following data showsthat the polyamic film deposited at 127° C. was etched by the water, andthe polyimide film deposited at 160° C. and the polyamic acid filmdeposited at 127° C. and subsequently cured to form polyimide were notetched by the water:

TABLE III Deposition at 127° C. Time (s) in H₂O Start Thickness (nm) EndThickness (nm) Δ (nm) 1 33.20 7.10 26.10 5 33.12 9.27 23.85 10 33.077.52 25.55

TABLE IV Deposition at 160° C. Time (s) in H₂O Start Thickness (nm) EndThickness (nm) Δ (nm) 10 41.10 40.87 0.23 20 40.72 39.89 0.83 60 40.1840.63 −0.45

TABLE V Deposition at 127° C., followed by treatment with argon plasma(200 W, 2 min) Time (s) in H₂O Start Thickness (nm) End Thickness (nm) Δ(nm) 10 40.05 41.33 −1.28 120 39.96 40.85 −0.89 300 39.40 41.02 −1.62

EXAMPLE 8

In another negative temperature difference experiment conducted onwafers patterned with trenches, 1,4-phenylenediisocyanate (PDIC) and DAHwere reacted in a sequential process to deposit a polyurea film on asubstrate with trench patterns. The trenches had variable pitches of 40and 50 nm with 25-35 nm openings. The first reactant PDIC was vaporizedat a temperature of 75° C., the PDIC gas line was maintained at 85° C.,and the substrate was maintained at 40° C. The second reactant DAH waskept at 45° C. Line flows of 450 sccm were used, and pulse/purge lengthsof 3/2 seconds and 8/7 seconds were used for PDIC and DAH, respectively.The resulting film was analyzed by tunneling electron microscopy (TEM).After 50 cycles, the TEM image showed that the film was thicker on thetrench bottom areas, and thinner on the side walls of the trenches (FIG.6A). The film thickness on a planar wafer grown using the sameparameters was 7 nm, the film thickness on the bottom of some trencheswas about 10 nm, and the film thickness on the sides of some trencheswas about 3 nm. The growth was thus proceeding faster in the bottomareas of the trenches, indicating bottom-up filling. After 215deposition cycles, the TEM analysis (FIG. 6B) showed seamless, bottom-upgap filling of the trenches with polyurea. The aspect ratio of thethree-dimensional features was decreased, exhibiting someself-planarizing behavior.

Although certain embodiments and examples have been discussed, it willbe understood by those skilled in the art that the scope of the claimsextend beyond the specifically disclosed embodiments to otheralternative embodiments and/or uses and obvious modifications andequivalents thereof.

What is claimed is:
 1. An apparatus for organic film deposition,comprising: a vessel configured to vaporize an organic reactant to forma reactant vapor; a reaction space configured to accommodate a substrateand in selective fluid communication with the vessel; and a controlsystem configured to: maintain the reactant in the vessel at or above atemperature A; maintain the substrate at a temperature B, wherein thetemperature B is between about 5° C. and about 50° C. lower than thetemperature A; transport the reactant vapor from the vessel to thesubstrate; and deposit an organic film on the substrate whilemaintaining the reactant in the vessel at or above the temperature A andmaintaining the substrate at the temperature B.
 2. The apparatus ofclaim 1, further comprising a gas line fluidly connecting the vessel tothe reaction space, wherein the control system is configured to maintainthe gas line at a temperature C, the temperature A being between thetemperature B and the temperature C.
 3. The apparatus of claim 1,wherein the control system is configured to deposit the organic film bytransporting a second reactant vapor to the substrate, the temperatureof the second reactant being less than temperature B.
 4. The apparatusof claim 3, wherein the control system is configured to transport thesecond reactant vapor to the substrate alternately with the reactantvapor in a sequential deposition process and to remove excess reactantvapors and byproduct between supply of the reactant vapor and the secondreactant vapor.
 5. The apparatus of claim 3, wherein the second reactantis a diamine.
 6. The apparatus of claim 5, wherein the second reactantis 1,6-diaminohexane (DAH).
 7. The apparatus of claim 1, wherein thereactant is a dianhydride.
 8. The apparatus of claim 7, wherein thereactant is pyromellitic dianhydride (PMDA).
 9. The apparatus of claim1, wherein the control system is further configured maintain a pressurewithin the vessel at a pressure higher than a pressure in the reactionspace.
 10. An apparatus for organic film deposition, comprising: avessel configured to vaporize an organic reactant to form a reactantvapor; a reaction space configured to accommodate a substrate and inselective fluid communication with the vessel; a gas line fluidlyconnecting the vessel to the reaction space; and a control systemconfigured to: maintain the reactant in the vessel at or above atemperature A; maintain the substrate at a temperature B, wherein thetemperature B is lower than the temperature A; maintain the gas line ata temperature C, the temperature A being between the temperature B andthe temperature C; transport the reactant vapor from the vessel to thesubstrate; and deposit an organic film on the substrate whilemaintaining the reactant in the vessel at or above the temperature A andmaintaining the substrate at the temperature B.
 11. The apparatus ofclaim 10, wherein the temperature B is between about 5° C. and about 50°C. lower than the temperature A.
 12. The apparatus of claim 11, whereinthe temperature B is between about 10° C. and about 30° C. lower thanthe temperature A.
 13. The apparatus of claim 10, wherein thetemperature B is in a range from about 100° C. to about 180° C.
 14. Theapparatus of claim 10, wherein the control system is configured todeposit the organic film by transporting a second reactant vapor to thesubstrate, a temperature of the second reactant being less than thetemperature B.
 15. The apparatus of claim 14, wherein the control systemis configured to transport the second reactant vapor to the substratealternately with the reactant vapor in a sequential deposition processand to remove excess reactant vapors and byproduct between supply of thereactant vapor and the second reactant vapor.
 16. The apparatus of claim14, wherein the second reactant is a diamine.
 17. The apparatus of claim16, wherein the second reactant is 1,6-diaminohexane (DAH).
 18. Theapparatus of claim 10, wherein the reactant is a dianhydride.
 19. Theapparatus of claim 18, wherein the reactant is pyromellitic dianhydride(PMDA).
 20. The apparatus of claim 10, wherein the control system isfurther configured maintain a pressure within the vessel at a pressurehigher than a pressure in the reaction space.